Capacitance sensors are used to implement a variety of useful functions including touch sensors (e.g., touch pad, touch dial, touch wheel, etc.), determining the presence of an object, accelerometers, and other functions. FIG. 1 illustrates a conventional capacitance sensor 100 including a relaxation oscillator, a reference clock, and a frequency comparator. The relaxation oscillator is coupled to drive a charging current (Ic) in a single direction onto a device under test (“DUT”) capacitor. As the charging current piles charge onto the DUT capacitor, the voltage across the capacitor increases with time as a function of Ic and its capacitance C. Equation 1 describes the relation between current, capacitance, voltage and time for a charging capacitor.CdV=ICdt  (Equation 1)
The relaxation oscillator begins by charging the DUT capacitor from a ground potential or zero voltage and continues to pile charge on the DUT capacitor at a fixed charging current Ic until the voltage across the DUT capacitor reaches a reference voltage (Vref). At Vref, the relaxation oscillator allows the accumulated charge to discharge or the DUT capacitor to “relax” back to the ground potential and then the process repeats itself. The relaxation oscillator outputs a relaxation oscillator clock signal (RO CLK) having a frequency (fRO) dependent upon capacitance C of the DUT capacitor and charging current Ic.
If capacitance C of the DUT capacitor changes, then fRO will change proportionally according to Equation 1. By comparing fRO of RO CLK against the frequency (fREF) of a known reference clock signal (REF CLK), the change in capacitance ΔC can be measured. Accordingly, equations 2 and 3 below describe that a change in frequency between RO CLK and REF CLK is proportional to a change in capacitance of the DUT capacitor.ΔC∝Δf, where  (Equation 2)Δf=fRO−fREF.  (Equation 3)
The frequency comparator is coupled to receive RO CLK and REF CLK, compare their frequencies fRO and fREF, respectively, and output a signal indicative of the difference Δf between these frequencies. By monitoring Δf one can determine whether the capacitance of the DUT capacitor has changed.
However, during the relaxation of the DUT capacitor, the discharge current is not fixed and therefore the relaxation phase generates an error. As frequency fRO of RO CLK is increased (e.g., by increasing charging current Ic) the relaxation error becomes a more significant portion of the overall clock cycle. As such, the relaxation error is a frequency limiting factor that places an upper bound on the operational frequency of capacitance sensor 100. Operating at lower fRO frequencies leaves capacitance sensor 100 more susceptible to low frequency noise. Furthermore, since the frequency fRO of RO CLK is unrelated to the frequency fREF of REF CLK, capacitance sensor 100 is susceptible to frequency wandering due to temperature drift.